1. Field of the Invention
This invention relates to a process for producing a polyaniline powder, particularly to a wastewater-recyclable process for producing a polyaniline powder.
2. Brief Description of the Related Prior Art
Referring to FIGS. 1 and 2, a conventional process of producing polyaniline comprise, a feed mixing step 11, a filtering step 12, a neutralizing step 13, a filtering step 14 and a drying and forming step 15. In the mixing step, an aniline monomer, an acid solution and an oxidizing agent (e.g. ammonium persulfate) are constantly mixed in a reactor 21 to undergo a reaction for 2-3 hours. Since the reaction is exothermic, heat is liberated so that a portion of reactants could form into undesirable polymers, thereby decreasing the yield of the desirable product, i.e. polyaniline. In order to eliminate the above drawback, the reactor 21 is cooled as shown in FIG. 2 so as to maintain the temperature of the reaction between 0-5.degree. C. After the reaction, solid polyaniline particles 16 and wastewater 17 are produced in the reactor 21.
In the filtering step 12, the wastewater 17 and the solid particles 16 produced via polymerization are separated through a filter (not shown). The wastewater 17 is usually discarded after separation. In the neutralizing step 13, the solid particles 16 are neutralized by a basic solution. In the second filtering step 14, the neutralized water contained in the solid particles 16 is removed by filtration. In the drying and forming step 15, the solid particles 16 obtained from the second filtering step are dried and formed into a powder. The powder as formed has a large particle size and is electrically conductive. Since the electrically conductive powder particles 16 can form lumps, they should undergo a grinding process for use as a conductive powder. Although the conductive powder 16 as formed can be used as an additive for preparing an electrical conducting material, the following problems are encountered with the conventional process.
(1) Difficult Temperature Control and By-product Formation
An exothermic reaction occurs during the mixing of the reactants. Since it is difficult to know when the heat of reaction is produced and to what level of temperature will be reached, stabilization of the reactants' temperature or accurate maintaining of the temperature between 0-5.degree. C. by mere use of a cooling water system is difficult. As long as the temperature falls out of the range of 0-5.degree. C., undesirable by-products are formed, thereby reducing the yield of the desired product.
(2) Pollution by Wastewater
Since aniline monomers are toxic, although they can be formed into acid salts by mixing with an acid solution in order to avoid toxicity, the amount of the acid solution is high so that a substantial amount of wastewater is produced during the mixing step. The wastewater contains unreacted acid and unreacted monomers, dimers and/or trimers and thus has contaminating colors. If the wastewater is dumped directly, serious pollution can result. Although this problem is solved via a wastewater treatment, the equipment and production costs are high.
(3) Large Particle Size
The conventional method of polymerizing aniline monomers comprises the mixing step, the filtering step, the neutralizing step and the second filtering step, and the solid particles as produced are large as shown in the attached FIG. 6. Accordingly, the solid particles must be ground to small-sized particles before mixing the same with a solvent in order to form a uniformly dispersed conductive paint. If the particle size is large, the powder cannot be easily suspended in the solvent, thereby increasing the difficulty in preparing an electrically conductive paint. Although this problem can be solved by using a suspension agent during the process of powder formation for preventing agglomeration of solid particles, the use of the suspension agent not only increases the material's cost but also results in the need to remove the residual suspension agent.
(4) High Equipment Cost
In addition to the need to provide a waste water treatment as described above, the conventional polymerization process further requires the equipment for water-cooling the reactor so as to maintain the temperature of the reactants, such as, aniline monomer, the acid solution and the oxidizing agent, within the range of 0-5.degree. C. in order to avoid formation other inferior polymerized products. The need to provide the cooling equipment is also one of the reasons that increase the cost of production.
(5) Unreacted wastes
When the aniline monomer, the acid solution and the oxidizing agent are mixed, the acid solution is generally not completely reacted, thus resulting in residual acid solution in the wastewater and wasting of the acid solution.